Intrinsically safe corrosion measurement and history logging field device

ABSTRACT

Corrosion measurement devices are described with electrical isolation and intrinsic safety barriers advanced corrosion measurement in a field transmitter for online corrosion monitoring or off-line corrosion data logging.

FIELD OF THE INVENTION

The present invention relates generally to corrosion measurement andmore particularly to low power field devices for measuring corrosion.

BACKGROUND OF THE INVENTION

Field installed transmitter devices have been widely employed in processcontrol situations to provide process variable information to controland/or data acquisition devices. Unlike laboratory measurementinstrumentation, field devices are constructed using sealed protectiveenclosures to withstand adverse environmental conditions inmanufacturing facilities, chemical processing plants, oil refineries andthe like, in which the device may be subjected to extreme temperaturesand humidity. Such devices are typically employed in distributed controlsystems for sensing temperature, fluid pressure, flow, and othervariables used to control an ongoing process and are generally connectedto other control equipment by a 4-20 mA control loop from which thetransmitter derives its power and through which the sensed processvariable is provided to the control system. Loop-powered transmittersare widely available for sensing various process variables, with thetransmitter generally being configured to vary the loop current from 4mA to 20 mA according to the measured process variable (e.g., with 0% ofthe process variable range being represented by 4 mA and 100%corresponding to 20 mA). Other transmitters offer digital communicationsaccording to standard protocols such as HART, etc. by which thetransmitter can send and receive data, commands, and other informationvia the control loop.

Many chemical processes involve storage or transport of fluids in orthrough pipes, tanks and other structures, wherein these structures maycorrode overtime due to contact with the transported or stored fluids.In these situations, it is desirable to ascertain the amount and rate ofsuch corrosion to allow informed evaluation of the structural integrityfor maintenance purposes and also to identify undesired or unexpectedcorrosivity levels in the fluids themselves, where the corrosion datamay be used to apply remedial measures such as inhibitor injectionand/or to ascertain and optimize the efficiency of such remedialmeasures. Other corrosion measurement applications include corrosion ofstructures exposed to non-fluids, such as corrosion of steel inconcrete, wherein corrosion-causing materials generally, whether solid,liquid, or gaseous are referred to as electrolytes. However, it is oftenimpractical to perform corrosion measurement using elaborate andexpensive laboratory grade instrumentation and measurement systems dueto the nature of most chemical processing environments and the size andlocation of pipelines and fluid holding tanks. In particular, suchexpensive systems are not adaptable to online measurement of pipeline orstorage tank corrosion conditions in real time. Moreover, onlinemeasurement devices must be able to operate on very low power budgets,such as obtainable from a standard 4-20 mA control loop or from batterypower. Field corrosion transmitters have recently been introduced toprovide corrosion measurement capabilities for these applications.However, conventional field corrosion transmitters have thus far beenunable to provide requisite levels of corrosion measurement accuracy andadaptability to measuring corrosion with respect to a wide variety ofstructure materials, transported fluid types, and temperatures whereby aneed exists for improved field transmitters for measuring one or morecorrosion related values.

SUMMARY OF INVENTION

Various aspects of the present invention are now summarized tofacilitate a basic understanding of the invention, wherein this summaryis not an extensive overview of the invention, and is intended neitherto identify certain elements of the invention, nor to delineate thescope thereof. Instead, the primary purpose of this summary is topresent some concepts of the invention in a simplified form prior to themore detailed description that is presented hereinafter. The inventionrelates to low power field devices for accurately measuring one or morecorrosion related values such as corrosion rate, localized corrosion(e.g., pitting), solution (electrolyte) resistance or conductivity, etc.in real time, and which can be employed in field situations not amenableto expensive and delicate laboratory instrumentation systems.

In accordance with one or more aspects of the invention, loop-poweredcorrosion measurement devices are provided for measuring or monitoringcorrosion of a structure exposed to an electrolyte. The devices comprisea power system to power the device using power from a 4-20 mA loop,battery, solar panel, etc. and a probe interface system with signalconditioning circuitry to interface with two or more measurementelectrodes situated in the electrolyte. The signal conditioning providesone or more excitation signals to the electrolyte via a first electrodeand includes sensing circuitry for sensing one or more corrosion-relatedelectrical signals such as currents, voltages, etc. via at least asecond one of the electrodes. In certain embodiments, a switching systemis provided with a plurality of analog switches operable according tocorresponding control signals to selectively interconnect circuitcomponents of the excitation and sensing circuitry and the electrodes ina plurality of different configurations to facilitate providingcorrosion related values such as electrolyte resistance or conductivity,corrosion rate, localized corrosion index, etc., using one or moremeasurement types including but not limited to solution (electrolyte)resistance (conductivity) measurement (SRM), harmonic distortionanalysis (HDA), linear polarization resistance measurement (LPR),electrochemical noise measurement (ECN), etc. The switching system mayalso operate to couple a driver amplifier of the excitation circuitrysuch that the excitation (auxiliary) and sensing (working) electrodesare in a feedback path of the driver amplifier, by which current flowingbetween these electrodes causes the voltage between the workingelectrode and a reference electrode to be the same as the appliedexcitation voltage. In certain embodiments, moreover, the switchingsystem connects a current limiting resistor of the sensing circuitry ina feedback path of a current to voltage converter circuit, so that thecurrent limiting resistance does not influence the current sensingoperation.

The devices also include a processing system, for example, including amicroprocessor, microcontroller, DSP, etc., operatively coupled with theprobe interface to control the excitation signals provided to theelectrolyte, to selectively configure and operate the sensingcomponents, and to compute one or more corrosion related values based onmeasured values from the sensing circuitry. The processor also operatesto store computed corrosion values in non-volatile memory for subsequentuploading. The corrosion value(s) may be provided as a process variableoutput in the form of a 4-20 mA signal on a connected control loop. Thedevice may also include a communications interface for HART or othertype digital communications allowing user configuration of the number ofmeasurements and measurement types in a given device cycle, and forproviding the user with corrosion related values via the loop or otherwired or wireless communications.

In accordance with other aspects of the invention, a rectifier system iscoupled with the sensing circuitry to provide a non-dc-free signal to ananalog-to-digital (A/D) converter based on a dc-free sensed signal,which may be part of a synchronous rectifier that alternates a polarityof a sensed signal in concert with an alternating polarity of anexcitation signal provided to the electrolyte. In this manner, the A/Dinput has a non-zero average value allowing sub-Nyquist sampling ratesand averaging to ascertain the value of the measured current. The use ofthe rectifier facilitates the provision of substantially dc-free squarewave excitation signals to mitigate inaccuracies and corrosionexacerbation associated with non-dc-free signals, while alsofacilitating conservation of device power so that the processing systemcan average a number of sensed current readings taken at low A/Dsampling rates in measuring electrolyte resistance/conductivity. In oneexample, the synchronous rectifier is comprised of a first switchtoggled by the processor to alternate the polarity of a sensed signaland a second switch that synchronously alternates the polarity of theexcitation signal to provide an essentially dc-free AC excitation signalto the electrolyte at an excitation frequency with the analog-to-digitalconverter receiving a non-dc-free signal which can be sampled at a muchlower sampling rate and averaged to measure the electrolyte resistanceaccurately. In one embodiment, the dc-free square wave excitation isprovided at a frequency of about 500 Hz or less, preferably about100-200 Hz, with the A/D converter sampling the sensed current at a rateof less than about ten samples per second.

In accordance with further aspects of the invention, the device includesan isolation barrier providing galvanic isolation of the electrodes fromthe 4-20 mA loop, by which the device is easily incorporated in a givenplant installation without the need for a separate isolation barrier.Other aspects of the invention involve providing intrinsic safetycircuitry in the device to protect against high currents and/or highvoltages, which may be a two-stage system, and which may includeresistors for protecting the electrodes.

According to still further aspects of the invention, the device canoperate as a stand-alone data acquisition and storage device, where theprocessor computes corrosion related values in each of a series ofdevice cycles and stores the computed corrosion related values forsubsequent retrieval by a user, where the values may be stored innon-volatile memory in the device. In this aspect, the device mayinterface with an external communications device through a control loopor other wired or wireless means to allow a user to configure the deviceand/or to retrieve stored computed corrosion related values therefromusing HART or other suitable communications protocol(s) wherein thedevice may store one or more day's worth of computed corrosion relatedvalues.

In accordance with further aspects of the invention, the electrolyteresistance measurement may be improved by dynamically adjusting the ACexcitation amplitude to better utilize the input range of the A/Dconverter, wherein the processing system provides the excitation signalat a first amplitude and selectively increases the excitation signalamplitude until a resulting sensed current signal exceeds a predefinedthreshold value.

According to still further aspects of the invention, the device isoperative to self-calibrate in order to compensate for offset errors inthe current sensing and rectifier circuitry, wherein the processingsystem controls the excitation circuit to initially provide noexcitation signal and causes the synchronous rectifier to beginalternating the polarity of a sensed current signal. The processorsamples the sensed current using the A/D converter while the synchronousrectifier operates with no excitation signal, and computes an offsetvalue based on one or more sensed current signal samples. The computedoffset value is then stored for subsequent use as an offset correctionin computing the electrolyte resistance or other corrosion relatedvalues.

In another aspect, the excitation is adjusted during HDA measurements tocompensate for differences in the electrodes, wherein the processingsystem measures a sensed voltage signal with no applied excitationsignal, stores the sensed voltage value as an offset, and thereaftercauses the excitation circuitry to provide sinusoidal excitation voltagesignals with the offset and computes the corrosion related value(s)using HDA based on harmonics of sensed current signals.

In accordance with other aspects of the invention, improved harmonicdistortion analysis (HDA) type measurements are facilitated usingsinusoidal AC excitation at a frequency of about 50 mHz or more, whereinthe device computes one or more corrosion related values based on morethan one cycle (preferably about 10 to 20 cycles) of the sensedsinusoidal current signal. This facilitates the measurement of corrosionrate using enough current samples to adequately perform Fourier analysisfor obtaining current harmonic readings without significantlylengthening the device measurement cycle time.

Still further aspects of the invention relate to dynamic algorithmchanges in which HDA measurements and computations are performed ifpossible, with automatic switchover to LPR measurements in highelectrolyte resistance situations or other conditions indicatingpossible inaccuracy in the HDA measurements. In this aspect, theprocessing system performs the HDA type measurements and computes aStern-Geary constant (B value) based on the harmonics of current signalssensed by the sensing circuitry in each of a series of device cycles.One or more plausibility tests are then performed with respect to thecomputed B value, such as ascertaining the relative sizes of thecomputed electrolyte (electrolyte) resistance R_(S) and a computedpolarization resistance R_(P) by determining whether the quantity(R_(S)/(R_(S)+R_(P))) is less than a threshold (e.g., about 0.1 in oneexample), determining whether the quantity (2I₁I₃−I₂ ²) is greater thanzero, and/or determining whether the computed B value is in a predefinedrange (e.g., about 10-60 mV in one embodiment). In one embodiment, ifthe plausibility test(s) indicates that harmonic distortion analysis islikely to yield an accurate corrosion related value, the processingsystem selectively computes the corrosion related value(s) usingharmonic distortion analysis with the computed B value, and otherwisethe corrosion value(s) is computed using LPR measurements with a userdefined or default B value.

In accordance with other aspects of the invention, the processing systemcomputes at least one statistical value using a running momentcalculation in deriving the corrosion related value(s) usingelectrochemical noise (ECN) measurements, thereby mitigating the need tostore large amounts of data and reducing the number of requiredcomputations in each device cycle.

In a related aspect, the processor is operative to compute a localizedcorrosion index based on a standard deviation of sampled current signalsand on an rms of the sampled current signals, where the standarddeviation and rms are both based on the running moment calculation.

Another ECN related aspect of the invention involves effectivelyshorting auxiliary and working electrodes by connection thereof to acircuit virtual ground during ECN measurements. In one embodiment, theprocessing system selectively reconfigures the switching components toconnect the auxiliary and working electrodes to a virtual ground in theprobe interface system, computes the corrosion related value(s) based onsignals sensed by the sensing circuitry using ECN.

Further aspects of the invention relate to the use of linearpolarization resistance (LPR) measurements in the device, where acalculated B value is employed rather than a predefined user B value.The computed B value, moreover, is preferably low pass filtered incertain embodiments, to facilitate noise immunity. In one embodiment,the processing system computes a B value based on harmonics of currentsignals sensed by the sensing circuitry in each of a series of devicecycles, and computes the corrosion related value(s) in a given devicecycle based on the computed B value and a current signal sensed by thesensing circuitry using a linear polarization resistance measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the invention in detail, which are indicative ofseveral exemplary ways in which the various principles of the inventionmay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the invention. Otherobjects, advantages and novel features of the invention will be setforth in the following detailed description of the invention whenconsidered in conjunction with the drawings, in which:

FIG. 1 is a perspective view illustrating an exemplary corrosionmeasurement device including a loop or battery powered transmitter withan associated probe and electrodes in accordance with one or moreaspects of the present invention;

FIG. 2 is a schematic diagram illustrating further details of thetransmitter of FIG. 1 including a digital system, a loop interface, anda probe interface;

FIG. 3A is a schematic diagram illustrating portions of the probeinterface system and the digital system in the exemplary transmitter ofFIGS. 1 and 2 including processor controlled excitation circuitry,sensing circuitry, and an analog switching system for programmaticreconfiguration of the device for a variety of different corrosionmeasurements;

FIG. 3B is a schematic diagram illustrating further details of theisolation circuitry in the loop interface system of the exemplarytransmitter of FIGS. 1 and 2 including an isolation transformer and atwo stage intrinsic safety barrier;

FIG. 4 illustrates a table showing several exemplary switching systemconfigurations for SRM, HDA, LPR, cell offset voltage, and ECNmeasurements in the device of FIGS. 1-3B;

FIG. 5 is a partial sectional side elevation view schematicallyillustrating the probe and electrodes of the measurement deviceinstalled in a pipe or storage structure with the electrodes exposed toa transported or stored electrolyte for corrosion measurement;

FIG. 6 is a simplified schematic diagram illustrating an equivalentcircuit for one of the electrodes and the measured electrolyte in theinstallation of FIG. 5;

FIG. 7 is a graph illustrating exemplary excitation waveforms applied tothe measured electrolyte by the excitation circuitry in a multi-phasemeasurement cycle of the device of FIGS. 1-6 including a substantiallydc-free 200 Hz square wave for electrolyte resistance measurement, a 100mHz sine wave for HDA and LPR measurements and an ECN portion with noexcitation;

FIG. 8 is a graph further illustrating the substantially dc-free squarewave excitation signal used in the device for electrolyte resistancemeasurements;

FIG. 9A is a flow diagram illustrating exemplary operation forelectrolyte (solution) resistance measurement (SRM) using dynamicexcitation amplitude adjustment in the device of FIGS. 1-6;

FIGS. 9B-9D are graphs showing voltage and current plots of square waveexcitation voltages and corresponding measured average currents fordifferent excitation waveform amplitudes during dynamic amplitudeadjustment in the device of FIGS. 1-6;

FIG. 10A is a graph showing a plot of an exemplary square wave voltageexcitation signal applied at about 200 Hz and two exemplary asynchronousA/D converter samples using a low sample period of about 300 msec;

FIG. 10B is a graph showing excitation voltage and sensed current plotsat the two exemplary sample times in FIG. 10A;

FIG. 10C is a flow diagram illustrating exemplary operation for onlinecurrent amplifier offset measurement in the device of FIGS. 1-6;

FIG. 11 is a flow diagram illustrating operation of the device fordynamic algorithm change for HDA or LPR measurement includingplausibility testing of a computed B value in the device of FIGS. 1-6;and

FIG. 12 is a flow diagram illustrating exemplary offset measurement andexcitation signal adjustment for HDA corrosion measurement in the deviceof FIGS. 1-6.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, several embodiments or implementations ofthe present invention are hereinafter described in conjunction with thedrawings, wherein like reference numerals are used to refer to likeelements throughout, and wherein the various features and plots are notnecessarily drawn to scale. The invention relates to programmable lowpower corrosion measurement field devices for providing corrosionmeasurement and monitoring using one or more advanced corrosionmeasurement types to provide conductivity, general corrosion, and/orlocalized corrosion values for real time corrosion monitoring and/oroff-line corrosion data logging which may be employed in distributedcontrol systems connected by a standard 4-20 mA control loop or othercommunicative means, or which may act as stand-alone devices with thecapability of downloading stored corrosion data to a user communicationsdevice.

Referring initially to FIG. 1, an exemplary field corrosion measurementdevice 2 is illustrated in accordance with one or more aspects of theinvention, including a transmitter head 4 that houses theprocessor-based electronic circuitry as described in greater detailhereinafter, along with and a probe 6 and a set of three electrodes 8which are preferably made of a material matching that of a metalstructure into which the device 2 is installed for corrosionmonitoring/measurement, where the electrodes 8 are immersed or embeddedin the solution or other electrolytic solid, gas, or liquid stored ortransported in the installed structure, such as a pipeline, storagetank, or other structure of interest. The transmitter housing 4 and theprobe 6 are constructed of environmentally protective materials to allowuse of the device 2 in field applications such as for online corrosionmonitoring to generate process variables for corrosion rate, localizedcorrosion index (degree of corrosion localization), and/or electrolyteresistance (conductivity). In operation, the probe 6 is mounted to astructure of interest with the electrodes extending into the interior ofa pipe or fluid chamber so as to be exposed to a corrosion processtherein.

As set forth in greater detail below, the device 2 can perform a numberof different corrosion related measurements, including measuringpolarization resistance RP, solution resistance RS, and electrochemicalnoise measurements. Linear polarization resistance (LPR) measurement isa simple technique that works in most situations, in which a set voltageis applied to the test electrode and the resulting current is measuredto derive a value for the polarization resistance R_(P). The corrosioncurrent I_(corr) is then calculated from the polarization resistance RP(=applied voltage/flowing current) as I_(corr)=B/R_(P), where B is theStern Geary constant. LPR is affected by any significant solutionresistance R_(S) present (appears in series with R_(P)) and also byuncertainty in knowledge of the B value, and is generally quite robustand immune to noise etc. especially if done using a sine wave excitationas in the embodiments illustrated and described further below. Harmonicdistortion analysis (HDA) calculates I_(corr) directly from theharmonics of the resulting current (if the applied voltage is a puresine wave), and does not require knowledge of a B value. Since I_(corr)from LPR and from HDA should theoretically be the same, it is possibleto calculate B from the harmonic information used in HDA computationsbased on a known or measured R_(P). HDA is also affected by solutionresistance R_(S), which tends to ‘linearize’ the response, whereinharmonic distortion is the result of non-linear R_(P), and not of R_(S)which appears in series with R_(P). Since the harmonics can be of lowlevel, the measurement result (I_(corrharm)) from HDA can be disturbedby noise from one measurement to the next, and if the B value iscalculated from a noisy I_(corr) and from R_(P), the computed B valuewill also be noisy. To address this situation, the values computed forI_(corrharm) could be filtered or averaged, but this would slow down theresponse time of the device 2 and inhibit the ability to detect changesof the corrosion attack. To address these competing goals, the device 2calculates an instantaneous B value for each measurement (from R_(P) andI_(corrharm)), and averages the B fluctuations by digital low passfiltering, wherein the filtered computed B value can be used with theinstantaneous R_(P) to calculate I_(corr). All the perturbativetechniques for SRM, LPR, and HDA provide an average corrosion rate ofthe test electrode and are not very sensitive to localized corrosion.ECN, however, is also possible in the device 2, which provides anon-perturbative measurement technique (no externally appliedexcitation), which provides an indication of any localized corrosion.ECN, moreover, can detect electrochemically caused localized attack(pitting, crevice corrosion, etc.) as well as localized attack caused bymechanical damage to any protective oxide film naturally formed on thetest electrode—such as cavitation damage, erosion damage, stresscorrosion cracking etc.

Referring also to FIG. 2, the electronics of the transmitter 4 areschematically illustrated, which include a loop interface 10 withgalvanic isolation and intrinsic safety (IS) barrier circuitry 12through which the device 2 interfaces with a standard 4-20 mA controlloop 11, and a power system 14 that provides internal device powerderived from either current from the control loop 11 or alternativelyfrom a battery 13, solar panel (not shown) or other source. The loopinterface 10 further includes a communications interface 16 operativelycoupled with a processor 22 of a digital system 20 and with the controlloop 11 to allow the processor 22 to communicate using HART or othercommunications protocol(s) with an external communications device (notshown) by which a user may configure or program the device 2 and/or mayretrieve stored computed corrosion related values from the device 20.The exemplary loop interface 10, moreover, includes a dedicateddigital-to-analog converter (DAC) 10 for controlling the current in theloop 11 so as to allow the processor 22 to control current in the loopto indicate a measured/computed process variable (e.g., loop currentlevel between 4 and 20 mA corresponding to corrosion rate, localizedcorrosion index, conductivity, etc.) and also provides for FSK or othertype modulation of the loop current to perform digital communicationsvia the loop 11 or other wired or wireless communications meansaccording to a suitable protocol such as HART, etc.

The device 2 also includes a digital system 20 comprising a processingsystem 22, which can be any form of processing circuitry such as amicroprocessor, microcontroller, digital signal processor (DSP),programmable logic, etc., by which the various functionality describedherein can be accomplished. The digital system 20 includes one or moreforms of memory 24, in particular, non-volatile memory such as flash,FRAM, etc., and may include an analog-to-digital converter (A/D) 26,wherein the A/D 26 and/or the memory 24 may be separate components orcircuits, or may be integrated in the processor 22.

The corrosion measurement device 2, moreover, includes a probe interfacesystem 30 with signal conditioning circuitry 34 to interface with aplurality of measurement electrodes 8 situated in the electrolyte to bemeasured. In the illustrated implementation, moreover, the probeinterface 30 includes a second DAC 32 for generating excitation signalsto be applied by the signal conditioning circuitry 34 to at least one ofthe electrodes 8 for certain measurement types as described furtherbelow, wherein the excitation DAC 32 may alternatively be located withinthe digital system 20 and may optionally be integrated with theprocessor 22. The signal conditioning circuitry 34 comprises excitationcircuitry 34 a that provides excitation signals according to the outputof the DAC 32 to the electrolyte via a first electrode E1, also referredto as an auxiliary electrode, and sensing circuitry 34 b is provided tosense one or more corrosion-related electrical signals, such asvoltages, currents, etc., via one or both of the other electrodes E2and/or E3, wherein the second electrode E2 is referred to herein as areference electrode used for sensing voltage signals in the electrolyte,and the remaining electrode E3 is referred to as a work or workingelectrode for sensing current signals with respect to corrosion rate,wherein the reference and/or auxiliary electrodes E2, E1 can be made ofinert material. The signal conditioning system 34, moreover, includes aswitching system 34 c with a plurality of analog switching componentsallowing processor controlled reconfiguration of the various componentsof the excitation circuitry 34 a and the sensing circuitry 34 b and theelectrodes 8 in a plurality of different configurations.

Referring also to FIGS. 3A, 3B, and 4, further details of certaincomponents of the probe interface system 30 and the digital system 20are illustrated, including the excitation circuitry 34 a, sensingcircuitry 34 b, and the switching system comprised of four analogswitching devices 34 c labeled U13-U16 in FIG. 3A. Each of the analogswitches U13-U16 has two switching states, indicated in the figure as a“0” state and a “1” state, wherein the processing system 22 providescorresponding switching control signals CS13-CS16 to control the stateof each switch 34 c. The analog switches U13-U16, moreover, can have athird operational state controlled by a chip select input (not shown) inwhich the switch terminal is disconnected from either of the poleterminals. The switches U13-U16 are thus coupled for processorcontrolled interconnection of the components of the excitation andsensing circuits 34 a and 34 b to reconfigure the corrosion measurementdevice 2 in a number of different corrosion measurement arrangements,wherein FIG. 4 shows a table 70 illustrating the switch settings orstates for SRM, HDA, LPR, Cell Offset Voltage, and ECN measurementoperation of the device 2. The exemplary device 2 may be programmed by auser to operate in any single one of the measurement modes in FIG. 4 ormay perform measurements in any combination of two or more of the listedmeasurement types in each of a series of device cycles, whereby thecorrosion transmitter 2 is easily configured to accommodate anycorrosion measurement or monitoring application. In this regard, theloop interface 30 allows for optimum interconnection of the signalconditioning circuitry and the electrodes 8 with respect to impedanceand accuracy within the tight power constraints imposed by loop orbattery power sources by minimizing the number of amplifiers and othercomponents while providing advanced performance in terms of corrosionmeasurement and monitoring using ECN, HDA, SRM, LPR and Offset in asingle user programmable device 2.

The processor 22 controls the excitation DAC 32 during each measurementperiod to provide suitable excitation to the cell via the excitationcircuitry 34 a, the first (auxiliary) electrode E1, and the switchingsystem 34 c, and also operates the measurement A/D 26 to obtaincorresponding measurements of cell voltages and/or currents via thesensing circuitry 34 b, the switches 34 c, and the reference and workingelectrodes E2 and E3, respectively. The electrode couplings are madethrough the probe 6 with resistors R49-R51 and filter network R54-R56,C56, C57, and C58 forming the connection to the excitation and sensingcircuitry 34 a and 34 b. In the scenarios described below, the device 2performs a series of measurements in each device cycle throughcontrolled switching of the devices U13-U16. In the illustrated device2, moreover, certain of the selectable measurement types (e.g., SRM,HDA, and LPR) involve application of excitation signals, while others(e.g., ECN) do not, wherein general corrosion is computed using HDA orLPR measurement types, electrolyte resistance or conductivity ismeasured using SRM techniques, and ECN measurements are used incomputing localized corrosion index values. In general, the excitationsignals (if any) are applied to the auxiliary electrode E1 as voltagesignals provided by the DAC 32 in either a first polarity using a firstamplifier (e.g., opamp) U12A directly through the “0” state path of theswitch U13 or in an opposite second polarity via an inverter configuredamplifier U12B through the “1” state of the switch U13 with a driveramplifier U10A providing a corresponding output voltage to the auxiliaryelectrode E1 through the “0” state path of the switch U16 and a resistorR61. In these configurations, moreover, the electrodes are in thefeedback loop of the driver amplifier U10A of the excitation circuitry34 a, whereby current flowing between the auxiliary and workingelectrodes E1 and E3 will cause the potential between the referenceelectrode E2 and the working electrode E3 to be the same as the appliedexcitation signal voltage. In certain operational configurations,moreover, no excitation is applied, wherein the switching systemelectrically isolates the auxiliary electrode E1 from the excitationcircuitry 34 a while the processing system 22 samples voltage signalsensed across E2 and E3 by the sensing circuitry 34 b.

The return current resulting from any applied excitation voltage signalsflows through the working electrode E3 in the exemplary three electrodepotentiostatic measurement configuration, wherein the sensing circuitry34 b senses such currents via a current sense amplifier U9A forming acurrent to voltage converter with a current limiting resistor R57current limiting resistor in a feedback path of the current to voltageconverter, by which the current limiting resistance R57 (e.g., 1 kOHM inone embodiment) does not influence the current sensing operation. Theresistance R57 of the current to voltage converter introduces a nearzero voltage drop into the current path, and the value thereof may beselected to provide a gain according to requirements high signalsensitivity and wide dynamic range, and to protect the inverting inputof U9A against overloads and is situated within the feedback loop of U9Aso as to eliminate the effect of the resistance R57 on the currentmeasurement. This current to voltage converter of the sensing circuitry34 b is used for sensing current in HDA and ECN measurements, and isalso used in combination with a synchronous rectifier in measuring thepolarization resistance LPR.

The current to voltage converter amplifier U9A provides an output toeither an inverting input or a non-inverting input of amplifier U8A forthe “0” and “1” states of the switch U15, respectively, where the outputof U8A provides one of two inputs to the A/D converter 26 for currentsensing. The current sense polarity switch U15 may thus be operated as arectifier for certain measurement types to achieve toggled switching viathe control signal CS15 from the processor 22. In this regard, when theexcitation polarity switch U13 and the current sense polarity switch U15are operated synchronously (by controlled switching of control signalsCS13 and CS15 by the processor 22), these analog switching componentsconstitute a synchronous rectifier used in certain embodiments formeasuring the electrolyte (solution) resistance R_(S) (SRM mode). Thecurrent sensing components, moreover, are employed without toggling ofthe polarity switch U15 for measurement of sensed currents from theworking electrode E3 in performing HDA, LPR, and ECN measurements in thecorrosion measurement device 2. The sensing circuitry 34 b furtherprovides voltage sensing capability with an amplifier U7A driving thesecond analog input of the A/D 26 for sensing the voltage at thereference electrode E2 through a high impedance path R59, which iscompared with a reference voltage VREF 31 using amplifier U5A.

The A/D 26 can thus obtain and convert analog voltage and current valuesunder control of the processor 22 and then provides digital values forthese measurements to the processor 22. The A/D converter 26, moreover,can be any suitable conversion device, such as a delta-sigma modulatorbased converter in one embodiment, and is preferably operated at arelatively slow conversion rate. For example, the A/D 26 in theillustrated embodiments is operated to obtain measurement samples of thevarious corrosion related sensed signals at a sample rate significantlylower than the excitation signal frequency, such as less than about 10samples per second, for example, sampling once every 300 msec in oneembodiment, in order to remain within the power budget of the powersystem 14 for loop or battery powered implementations. The processingsystem 22 is thus operatively coupled with the probe interface system 30to control the excitation signals provided to the electrolyte by theexcitation circuitry 34 a and to provide control signals CS13-CS16 tothe switching system 34 c to selectively reconfigure the switchingcomponents U13-U16 to perform a plurality of different corrosionmeasurement types and to compute at least one corrosion related valuebased on received measured values from the sensing circuitry 34 b.

The circuitry of the transmitter 4 thus provides for application ofexcitation signals to the electrodes 8 via the probe 6, signalconditioning for sensing electrode signals, analog-to-digital conversioncircuitry 26, and processing components 22 for controlling theapplication of excitation signals, obtaining signal measurements, andperforming signal analysis to generate corrosion rate, electrolyteresistance or conductivity, and/or localized corrosion index processvariables, in addition to 4-20 mA loop interfacing including support forHART communications. The device 2 may be configured to generate the 4-20mA signal in the control loop 11 representative of a process variable,which may be a general corrosion rate in mils or mm per year indicatingthe current rate of an ongoing generally continuous corrosion process,an electrolyte resistance or conductivity value, or a pitting orlocalized corrosion factor, which is unitless and which represents ameasure of the degree of localization in a process that may have a lowcorrosion rate, but which may lead to small but eventually deep holes inthe pipe or tank material that reduce the material strength. The device2 may also be configured to report one or more of these values throughdigital communications, either periodically or in response to datarequests from another control entity on the loop 11 or from a usercommunication device.

Referring now to FIGS. 1-3B, the device 2 includes isolation andintrinsic safety (IS) barriers 12 providing galvanic isolation of theelectrodes E1-E3 and the circuitry of the device 2 from the 4-20 mAloop. As shown in FIG. 3B, the current from the 4-20 mA loop 11 passesthrough an input stage of a primary safety area 12 a with a fuse F1, asurge protector N1 and resistor R3 and a rectifier 12 a 1, followed byan inverter 12 a 2, which provides an input to an isolation transformerT1. The isolated output of the transformer T1 provides an input to asecondary isolated area 12 b, including a voltage protection circuit 12b 1 comprising voltage limiting zeners N6-N9 and current limitingcircuits formed of transistors P5-P8 and resistors R17, R21, R29-30,R34, R35, and capacitor C34. The output of this first intrinsic safetybarrier stage 12 b provides an input to a second IS barrier stage 12 cincluding further voltage limiting zeners N10-N15 thereby furtherlimiting the possible voltage seen by a loop controller circuit 15. TheIS protection of the device 2 also provides 1 KOHM protection resistorsR57-R61 to protect the electrodes E1-E3. In operation, the measuredelectrolyte and the electrodes E1-E3 are typically connected to an earthground, whereby the front end of the probe interface circuitry 30 isalso grounded through a low impedance path. The isolation circuitry 12,and particularly the provision of the isolation transformer T1 mitigatesor avoids potential ground loop problems through the 4-20 mA controlloop 11 where multiple devices are installed in the same tank orpipeline. Thus, the device 2 may be used without external isolationcomponents, thereby saving installation costs. Moreover, the isolationtransformer T1 constitutes a component of the exemplary integralexplosion prevention (intrinsic safety or IS) barrier. This dual stageIS circuitry 12 b and 12 c allows use of the device 2 in applicationsrequiring intrinsic safety without the need for external IS circuitry,thereby further reducing installation costs. In these applications, forinstance, the housing 4 (FIG. 1) is constructed as an explosion proofcertified enclosure (e.g., for U.S.) or indicated as “EX d” for Europe,and the provision of the internal IS circuitry in the device 2 allowsconnection thereof to a non-IS loop 11. In this manner, the exemplarydevice 2 effectively “converts” the Ex d or Explosion proof installationmethod to an IS protection for the electrodes 8 and the probe 6 (FIG. 1)without requiring an additional barrier.

Referring also to FIGS. 5-7, in operation, the probe 6 is installed withthe electrodes 8 being immersed in a electrolyte 50 being transported ina pipe or other metal structure 40, as illustrated in FIG. 5, whereinthe electrodes may be of any geometry, including being flush mounted,and need not be in-line with one another, wherein the illustration ofFIG. 5 is a schematic representation only. FIG. 6 shows an equivalentelectrical circuit 60 for one of the electrodes E1 and the measuredelectrolyte 50 in the installation of FIG. 5, wherein the electricalcircuits of the other electrodes E2 and E3 are equivalent to the circuitrepresented in FIG. 6. As shown in FIG. 6, the electrode/electrolytecircuit 60 includes the series combination of an internal cell voltageV_(C) and a polarization resistance R_(P) which are in parallel withelectrochemical double layer capacitance Cdl between the electrode E1and the electrolyte 50, where the electrolyte 50 has a resistance R_(S)which is the subject of SRM measurements. As shown in an excitationsignal graph 100 of FIG. 7, the signal measurements in one possibleconfiguration of the transmitter device 2 are performed in threemeasurement periods 101, 102, and 103 that may alternatively be in anyorder in each of a series of device cycles, or the device 2 may beprogrammed to perform only one measurement per device cycle, or anycombination of two or more measurement types in a given device cycle. Inthis configuration, the SRM measurement proceeds initially to providethe solution resistance value R_(S), which is then used in the LPR orHDA measurements in determining the corrosion rate to correct for anyerrors in the computation of the polarization resistance R_(P), as theseresistances R_(S) and R_(P) are essentially in series as shown in FIG.6.

In the first measurement phase 101 of the exemplary configuration shownin FIG. 7, a synchronous rectifier is operated initially in period 100 afor offset measurement as described further below, after which theamplitude of the AC excitation signal is dynamically adjusted in period100 b. A relatively high frequency ac excitation signal is appliedthereafter in portion 101 for solution resistance/conductivitymeasurement, followed by a gap 100 c in which offsets are measured dueto imbalances caused by non-identical electrodes 8. In the first phase101, moreover, the device 2 advantageously applies an AC waveform with amean of zero (substantially free of DC offset) to avoid polarizing theworking electrode interface. Moreover, in the exemplary device 2, theDAC 32 and processor 22 are operated at low speeds (for powerconservation), wherein the DAC output during SRM is set to a given dclevel and the output polarity is switched using the switching system 34c to generate a bipolar square wave excitation signal for SRMmeasurements. In order to minimize the effects of possible small DC cellcurrents created by the SRM measurements in phase 101, the duration ofthe phase 101 is set to be as short as possible and the gap period 100 cis provided with no polarization following the SRM measurements andbefore the LPR measurement in phase 102, thereby allowing the workingelectrode interface to depolarize.

In the first phase 101, the electrolyte (solution) resistance R_(S) (andhence the electrolyte conductivity 1/R_(S)) is measured using highfrequency square wave excitation. In the second portion 102, the device2 applies a lower frequency sine wave excitation voltage and measurescurrent and the associated harmonics for determining the corrosion rateusing LPR and/or HDA techniques. In the third portion 103, no excitationis applied, and the device measures electrochemical noise using ECNmeasurements for determining the localized corrosion index.

During the first portion 101 of the device cycle, the processor 22causes the switching system 34 c to configure the switches U13-U16 asshown in the SRM row of table 70 in FIG. 4, with U14 and U16 in the “1”switch states and with the synchronous rectifier operating with theswitches U13 and U15 being toggled synchronously under control of theprocessing system 22 to provide a square wave excitation/current senserectifier frequency of less than about 500 Hz, preferably about 100-200Hz, wherein the graph 100 in FIG. 7 shows operation at a frequency ofabout 200 Hz in the first measurement period 101. It is noted in theequivalent circuit of FIG. 6 that application of a relatively highfrequency (e.g., above about 50 Hz for example) will effectively shortthe upper leg because of the capacitance Cdl, wherein the resulting ACcurrent sensed via the working electrode E3 will be inverselyproportional to the electrolyte resistance R_(S). Other waveforms couldbe used for the SRM measurement, such as sine waves, square waves, etc.The illustrated SRM measurement in period 101 involves provision of thesquare wave excitation voltage at the auxiliary electrode E1 togetherwith the measurement by the sensing circuit 34 b and the A/D 26 of thecell current sensed at the working electrode E3, wherein the DAC 32(FIG. 3A) provides a DC output signal at a level controlled by theprocessor 22 with the switching of U13 alternating the polarity of theapplied excitation voltage at the excitation frequency controlled by theprocessor 22 via control signal CS13. The resulting sensed cell currentat the working electrode E3 will also be a square wave at the excitationfrequency. The processor 22 also operates the current sense polarityswitch U15 via signal CS 15 to toggle at the same frequency, whereby thesensed AC current signal will be rectified to present a rectified inputsignal to the A/D converter 26. In order to conserve power, theprocessor 22 controls the sampling of the A/D converter 26 at a muchlower frequency, such as about 3.3 Hz in one embodiment. The processor222 thus obtains many readings of the sensed current and averages thesereadings to compute the average sensed current, which is then used tocompute the electrolyte resistance R_(S).

Referring also to FIG. 8, operation of the synchronous rectifier allowsthe provision of a substantially dc-free excitation signal to theauxiliary electrode E1 so as not to exacerbate corrosion in the cell,while the rectification of the sensed current signal via U15 allows theA/D converter 26 to be operated at a low sample rate and hence toconserve power, while taking enough samples to allow the processingsystem 22 to obtain an accurate average current value, wherein absentsuch rectification, the average current value would be zero or nearzero. In this regard, it is noted that application of dc voltages to theauxiliary electrode alters the electrochemistry of the corrosion processbeing measured and may therefore interfere with any subsequent corrosionrate measurements. In addition, the rectification in the current sensecircuitry will effectively eliminate any dc in the sensed currentattributable to non-identical electrodes 8 by essentially chopping suchdc component into an ac component with a mean value of zero. Moreover,the synchronous rectification also operates to reject interference atfrequencies other than the switching frequency. FIG. 8 illustrates onepossible substantially dc-free square wave excitation signal waveformapplied during the first measurement period 101 by operation of the DAC32 and the synchronous rectifier, having an amplitude of approximately+/−20 mV, wherein the DAC 32 of FIG. 3A provides a substantiallyconstant dc value which is then polarity switched by toggling of theswitch U13 to create the excitation waveform at the auxiliary electrodeE1. The device 2 thus advantageously provides a non-intrusive dc-freesquare wave excitation signal in the first measurement period 101, whileproviding for synchronous rectification allowing slow sampling of thesensed current in performing SRM measurements within the limited powerbudget of loop or battery power along with rejection of dc and noise.

Referring also to FIGS. 9A-9D, the device 2 preferably adjusts themagnitude or amplitude of the square wave excitation signal in SRMmeasurements either at predefined time periods, or at the beginning ofeach SRM measurement period 101. This facilitates improved usage of theinput range of the A/D converter 22, thereby facilitating improvedaccuracy in the measured current samples, and in the computed averagecurrent value and hence improved electrolyte resistance (orconductivity) measurements. A process 120 in FIG. 9A illustrates thisexemplary operation, wherein the SRM cycle 101 begins at 122 and arelatively high frequency square wave excitation signal is provided at124 to the auxiliary electrode E1 at a first (e.g., low) peak-to-peakamplitude. In one example, the square wave frequency is about 200 Hz,although other values may be used, preferably about 500 Hz or less.FIGS. 9B-9D illustrate graphs 140, 144, 150, 154, 160, and 164 showingvoltage and current plots of square wave excitation voltages andcorresponding measured average currents for different excitationwaveform amplitudes according to the process 120 in FIG. 9A. In thefirst plot 140 of FIG. 9B, a square wave of about 200 Hz is applied at arelatively low first amplitude 142. The average current is measured at126 in the process 120, for instance, by taking a plurality ofmeasurements with the A/D 26 using the synchronous rectifier operationas described above or using other suitable techniques for measuring anaverage current value. A determination is made at 128 as to whether theaverage current value thus obtained exceeds a predetermined thresholdTH, where any suitable threshold may be used by which a decision can bemade regarding optimal usage of the A/D input range. In one example, thethreshold is related to about half of the A/D input range although othervalues can be used.

If the measured current does not exceed the threshold TH (NO at 128), asshown in the current plot 144 of FIG. 9B, the excitation signalamplitude is increased at 130 (e.g., by increasing the output of the DAC32 under control of the processing system 22), and the process 120 ofFIG. 9A returns to again measure the average current at 126. Thissituation is shown in plots 150 and 154 of FIG. 9C, wherein the newexcitation signal amplitude 152 is greater than the initial amplitude142 of FIG. 9B. The new average current is compared with the thresholdTH at 128, and as seen in the plot 154 of FIG. 9C, this current is stillbelow the threshold TH. Accordingly, the process 120 of FIG. 9A againincreases the excitation amplitude at 130 to a level 162 shown in theexcitation voltage plot 160 of FIG. 9D. At this point, as shown in plot164 of FIG. 9D, the latest excitation amplitude 162 provides for aresulting sensed average current that is greater than the threshold TH(YES at 128 in FIG. 9A), and the process 120 of FIG. 9A continues to 132whereat the electrolyte resistance R_(S) is computed using the latestexcitation voltage amplitude value, and the SRM process in period 101 isfinished at 134. In this manner, the corrosion measurement device 2 isadapted to utilize the full extent of the A/D conversion range, whereinthe processing system 22 correlates the known latest excitation voltageamplitude with the latest measured and computed average current value at132 to compute the electrolyte resistance R_(S) and/or electrolyteconductivity. This adaptive adjustment of the excitation amplitudefacilitates the optimal usage of the available A/D resolution, andprovides for adaptation of the device 2 for applications having very lowor very high electrolyte conductivities without sacrificing accuracy.

Referring also to FIGS. 10A-10C, the device 2 also provides forcalibration for current amplifier offset to further refine the accuracyof the computed corrosion related values. In this regard, the usage ofthe synchronous rectifier described above in conjunction withasynchronous A/D sampling may lead to situations in which the measuredcurrent and the input to the A/D converter 26 increase slightly duringeach cycle of the square wave as shown in FIGS. 10A and 10B. The plot170 of FIG. 10A illustrates the 200 Hz square wave voltage excitationsignal employed in SRM measurements along with two exemplaryasynchronous A/D converter samples S1 and S2 at times T₁ and T₂,respectively, obtained using a long A/D sample period of about 300 msec.The graphs 172 and 174 in FIG. 10B show further details of the exemplaryportions of the excitation voltage and sensed current plots,respectively, at the two exemplary sample times T₁ and T₂ in FIG. 10A,wherein it is seen that the first current sample S1 is somewhat lowerthan the second sample S2 simply because these were sampled at differentpoints within the excitation cycle. In addition to these inaccuracies,offsets in the opamps U8A and U9A used to sense the current signals maycontribute to reduced accuracy in computation of R_(S), corrosion rate,and/or localized corrosion. Further inaccuracies may result from a dcoffset difference between the inverting and non-inverting paths of therectifier, the finite speed of the cell driver amplifier U10A, resistorsand capacitors on the probe inputs.

In order to mitigate these inaccuracies, the device 2 provides foronline current amplifier offset measurement, with an exemplary process180 being illustrated in FIG. 10C beginning at 182 by which the device 2automatically determines an online offset value based on a measuredcurrent amplifier offset while the synchronous rectifier components U13and U15 are toggled by the processor 22. At 184, the processor 22 causesthe DAC 32 to set the excitation signal to zero, and begins toggling thesynchronous rectifier components U13 and U15 via signals CS13 and CS15,respectively, at 184 with no applied excitation voltage, wherein therectifier components are switched via signals CS13 and CS15 at the samerate as normally used for SRM measurements as described above (e.g., atabout 200 Hz in on implementation). The processor 22 obtains a number ofsamples of the sensed current signal at 188 using the A/D 26 andcomputes an average current value at 190, which is then stored forsubsequent use as an offset in the above described SRM measurements, andthe online current amplifier offset measurement is finished at 192.Thereafter during the SRM measurements in period 101, the processor 22uses the stored offset to correct the current readings before computingthe electrolyte resistance value R_(S), so as to counteract the adverseeffects of offsets in the current sensing circuitry including amplifiersU9A and U8A and to compensate for sampling inaccuracies associated withthe synchronous rectifier operation and the asynchronous sampling of theA/D converter 26.

Referring now to FIGS. 3A, 3B, 4, 7, and 11, the device 2 also providesfor improved HDA and/or LPR measurement types, wherein FIG. 4 shows theswitching system configuration for these modes with respect to theswitch states of U13-U16 in FIG. 3A. The device 2 is thus configurableto compute a general corrosion rate I_(CORR) using LPR or HDAtechniques. Basic LPR measurements typically employ a default or userentered B-value, whereas the HDA approach involves calculation of aB-value and the corrosion rate at the same time according to measuredcurrent harmonics. However, the inventors have appreciated that theconventional HDA computations are not viable or robust in all corrosionapplications, wherein the device 2 operates to selectively employ one orthe other of these techniques (HDA or LPR) according to the results ofonline plausibility tests using the measured current harmonics andelectrolyte resistance.

The second exemplary measurement portion 102 in FIG. 7 illustrates theexcitation applied in this portion 102, in which a low frequency sinewave excitation voltage is applied to the cell via auxiliary electrodeE1 for LPR or HDA type measurements of current harmonics. In thesemeasurement types, the sinusoidal excitation signal is preferably at anexcitation frequency of about 50 mHz or more to allow for measurement ofcurrent samples over more than one cycle of the excitation signalwithout unduly lengthening the device cycle time. Thus, in certainpreferred embodiments, the excitation signal is provided at anexcitation frequency of about 100-200 mHz, wherein the example of FIG. 7uses an excitation frequency of about 100 mHz. Moreover, the processingsystem 22 in the preferred embodiments computes the corrosion relatedvalue(s) based on more than ten cycles, preferably about 20 cycles ofthe sensed sinusoidal current signal using harmonic distortion analysisor LPR in the second period 102. Thus, compared with conventionaldevices that measure current signals over only a single cycle with anapplied excitation of 10 mHz, the illustrated device 2 provides improvedability to perform discrete Fourier transforms in the processing system22 using data obtain across more than a single cycle, thereby improvingthe resulting corrosion related value computational accuracy. In thisrespect, the use of an exemplary excitation frequency of about 100 mHzwith current signals sampled over about 20 cycles yields significantimprovement, while only marginally increasing the duration of the secondmeasurement period 102 for HDA or LPR measurements.

In the second period 102 of FIG. 7, the low frequency sinusoidalexcitation causes a resulting sensed current signal having variousfrequency domain components, including a fundamental component at theexcitation frequency and second and third harmonic components that areused for the corrosion related value computations in the processor 22.This harmonic information is obtained by sampling the sensed currentsignal and conversion thereof to digital data by the A/D 26, with theprocessing system 22 performing a discrete Fourier Transform (DFT) togenerate a frequency domain spectrum for the sensed current. Asdiscussed above, the exemplary embodiments of the device 2 sample thesensed current signals over more than one excitation cycle, andpreferably use an excitation frequency greater than about 50 mHz,preferably about 100-200 mHz so as not to unduly extend the device cycletime, although the invention is not limited to these specific examples.From the DFT frequency domain spectrum, amplitudes of the fundamentaland various harmonics are obtained, and the harmonic measurement data isused in calculating the corrosion rate. In one possible implementation,moreover, the DFT is computed in concert with the sine wave excitationvoltage generation, wherein the sinusoidal excitation voltage isgenerated as a series of small steps by the DAC 32 (FIG. 3A) from amemory look-up table in the processing system 22 or the memory 24 (FIG.2), with the same look-up table being used for the DFT computations. Inthis regard, the exemplary table uses 96 steps per cycle to keep thesize of the table small, and also to allow division by 2, 3, and 4.

The DFT is done essentially ‘on the fly’ in the illustrated example, bymultiplication of the current sample value by an appropriate number fromthe look-up table. Furthermore, the exemplary DFT computation calculatesthe real and imaginary components at each of the harmonics, and employsthe modulus as the square root of the sum of the squares thereof (e.g.,(real²+img²)^(1/2)) as this is less sensitive to noise than selectingone or the other of the real and imaginary components. The fundamentaland the 2nd and 3rd harmonic are computed by multiplying the A/D valueby an appropriate value from the sine look-up table after each discretesine step, and adding the result to the appropriate accumulatorvariable, wherein this approach mitigates the need for intermediate datastorage. The output of the DAC 32 is preferably scaled using a resistivedivider R52, R53 to decrease the size of the smallest single bit step,where the values of R52 and R53 may preferably be selected to cover thewidest possible range of cell offset, while minimizing the single bitstep size, and the processing system 22 may ensure that the cell offsetand/or required perturbation amplitude do not exceed the availablerange. Furthermore, in the illustrated embodiments, sequence delays maybe provided to allow for the effects of step changes in the sine outputon the cell current to pass prior to cell current sensing/measurement bythe A/D 26.

The excitation frequency is preferably selected to be less than thetypical time constant of the corrosion process (e.g., Cdl in parallelwith R_(P)) but high enough for a reasonable measurement time to obtaindata from more than one excitation signal cycle. In this respect,sampling over a fairly large number of cycles at a somewhat higherfrequency mitigates the amount of sampled discontinuities. For instance,if fairly slow signal drifting is occurring, a discontinuity will besampled (e.g., a difference between the first and last samples) if datais only taken over a single cycle, where such discontinuity will resultin a Fourier spectrum with excessive harmonic content, thereby adverselyaffecting the HDA measurement technique. Moreover, averaging over morethan one cycle improves immunity to interference and noise.

In the illustrated implementation, the processing system 22 evaluatesthe following equations (1)-(3) in each device cycle using the harmonicdata obtained in the measurement period 102 to compute the corrosioncurrent I_(corr), from which the corrosion rate can be determined:I _(corrharm) =I ₁ ²/((48)^(1/2)*(2*I ₁ *I ₃ −I ₂ ²)^(1/2))  (1)B _(HARM)=(I _(corrharm)*Sine Amplitude)/I₁)−(R_(S)*I_(corrharm))  (2)I _(corr)=((B _(HARM) OR B _(USER))*I₁)/((Sine Amplitude)−(R _(S) *I ₁)), where I₁ is the fundamental component of the sensed current and I₂ andI₃ are the second and third harmonic components, respectively, SineAmplitude is the amplitude of the sinusoidal excitation voltage signalapplied in period 102, and B is the application specific corrosionprocess value in units of volts. Once the corrosion current I_(corr) iscomputed, this can be multiplied by constants relating to the specificelectrode size, the faraday constant, and the atomic weight of thematerial, to calculate the corrosion rate in mm or mils per year.

Referring also to FIG. 11, another feature of the exemplary corrosionmeasurement device is the computation of the B value B_(HARM) based onthe measured current harmonics I₁, I₂, and I₃ and the selective use ofLPR or HDA algorithms based on the calculated B_(HARM) value and thecomputed electrolyte resistance R_(S). In this embodiment, HDAmeasurements and computations are performed if possible, and if the HDAresults appear suspect based on one or more plausibility tests in agiven device cycle, the processing system 22 changes to LPR typemeasurements. In particular, the device 2 automatically performs one ormore of three types of tests to determine whether HDA computations arewarranted and selectively changes the algorithm to LPR in highelectrolyte resistance situations or other conditions indicatingpossible inaccuracy in the HDA measurements.

A dynamically changing HDA/LPR process 200 is shown in FIG. 11 beginningat 202 for the second period 102 in the exemplary device cycle of FIG. 7above, wherein the processor 22 causes the DAC 32 and the excitationcircuitry 34 a to provide a sinusoidal excitation signal to theauxiliary electrode E1 at 204 and measures the current signal sensed atthe working electrode E3 by the sensing circuitry 34 b at 206 using theA/D converter 26. The processor 22 performs a DFT to identify thecurrent harmonics I₁, I₂, and I₃ at 208 and then performs one or moretests at 210 to ascertain whether HDA corrosion measurements areplausible. In particular, a determination is made at 212 as to whetherthe quantity (2*I₁*I₃−I₂ ²) is positive. If not (NO at 212), the HDAtype measurement is deemed to be not plausible, since the square root ofthe tested quantity (2*I₁*I₃−I₂ ²) appears in the denominator of theabove equation (1). The process 200 continues to 230 in FIG. 11, whereatthe processing system 22 obtains a default or user provided B valueB_(USER) and employs this in the LPR corrosion current equation (3)above at 232 to compute I_(CORR) in the current period 102 whereafterthe cycle ends at 240.

If, however, the first tested quantity (2*I₁*I₃−I₂ ²) is found to bepositive (YES at 212), the process 200 proceeds to 214 where adetermination is made as to the relative size of the electrolyteresistance R_(S) compared to the polarization resistance R_(P) todetermine whether the harmonics are accurately measurable, wherein highR_(S) tends to linearize the cell response leading to low harmoniclevels. In the illustrated embodiment, the quantity(R_(S)/(R_(S)+R_(P))) is compared at 214 against a threshold, such asabout 0.1 in one example, and if less than the threshold (NO at 214),the processor 22 decides that HDA may be suspect and sets a flag at 215before proceeding to 216. Alternatively, the process may proceed to 230to switch to LPR operation after the flag is set at 215. If the test at214 does not indicate high R_(S) (YES at 214), the process proceeds to athird test at 216, 218 with the processing system 22 computingI_(CORRHARM) and B_(HARM) at 216 by evaluating the above equations (1)and (2) using the measured current harmonics I₁, I₂, and I₃ and low passfilters the computed B value B_(HARM). The computed B value B_(HARM) inthe illustrated example is low pass filtered digitally (e.g., movingaverage or other low pass type digital filtering performed by theprocessor 22), to remove any short term fluctuations and invalidreadings, thereby extending the device sensitivity in situations wherethe measured harmonics may be of very low amplitude.

A determination is then made at 218 as to whether the computed B valueB_(HARM) is in a specified presumed valid range between a minimum valueB_(MIN) and a maximum value B_(MAX), such as between about 10-60 mV inone example (e.g., or other range known to be viable for aqueouselectrochemistry). It is noted that the exemplary low pass filtering ofthe computed B value B_(HARM), such as a moving average or other digitalfilter, advantageously operates to remove any short term fluctuationsand occasional rogue readings, whereby the device sensitivity may beenhanced with respect to low amplitude harmonic situations by using thefiltered or smoothed computed B value. In one example, the filteredvalue B_(HARM) is computed as (1−X)*B_(HARM(n−1))+X*B_(HARM(n)), where Xin one implementation is about 0.05. If B_(HARM) is not in the testrange (NO at 218), the HDA technique is suspect, and the process 200proceeds to 230 and 232 as described above. Otherwise (YES at 218), theprocessing system 22 calculates the corrosion current at 220 using HDAtechniques by evaluating the above equation (3) using the computed Bvalue B_(HARM).

Yet another feature of the corrosion device 2 is the ability to utilizethe computed B value B_(HARM) (e.g., preferably low pass filtered) inperforming LPR type measurements instead of a predefined user B valueB_(USER). In one embodiment, the processing system computes a B valuebased on harmonics of current signals sensed by the sensing circuitry ineach device cycle according to the above equation (2) and computes thecorrosion related value(s) using equation (3) based on B_(HARM). Inaddition, the user may configure the device 2 for LPR measurements usinga user B value B_(USER), which may be obtained by any suitable meanssuch as correlating weight loss data from test coupons, electricalresistance probes or wall thickness measurements, with LPR readings,wherein the computed B value B_(HARM) may be monitored by a user or DCSto which the device 2 is connected. In this regard, observed changes inthe computed B value B_(HARM) may indicate changes in processelectrolyte composition changes or other process events of interest froma process control/monitoring perspective.

Referring also to FIG. 12, another feature of the device 2 is theadjustment of the sine wave HDA/LPR excitation signal to compensate fordifferences in the electrodes 8. In this regard, in the ideal cell withidentical electrodes 8, no net dc current would flow between theelectrodes over a whole cycle of a sine wave excitation, in which case,the electrochemistry of the working electrode E3 would not be disturbed.However, assuming non-identical electrodes 8, a goal is to ensure thatwhen no excitation is applied by the device 2, the current through theworking electrode E3 is zero. Since the electrodes 8 are in the feedbackloop of the driver amplifier U10A, the current flowing from theauxiliary electrode E1 to the working electrode E3 causes the potentialbetween the reference and working electrodes E2 and E3 to be the same asthat of the applied excitation.

In the example of FIG. 12, the processing system 22 switches the analogswitches at 254 to the states indicated for ECN measurement in the table70 of FIG. 4. Thus configured, the voltage signal at the referenceelectrode E2 is measured at 256 with no excitation, and is stored foruse as an excitation offset during the HDA measurements, whereupon theonline electrode offset measurement is finished at 258. Thereafter, theswitching system 34 c is switched at 260 by the processor 22 to the HDAconfiguration shown in the table 70 of FIG. 4, and the HDA measurementsare taken at 262 with the offset value added to the excitation signal bythe DAC 32 under control of the processor 22. In this manner, the device2 performs the HDA measurements during the second measurement period 102of FIG. 7 using the offset so as to compensate for any inaccuraciesotherwise attributable to differences between the electrodes 8. Bymeasuring the cell offset before the HDA is performed and adding themeasured offset to the applied sine wave, any currents that are causedby the electrode differences are effectively eliminated during HDAmeasurement, whereby the device compensates for physical differencesbetween the electrodes E1-E3 and thus increases accuracy and reliabilityof the HDA corrosion rate results. With respect to measurements at theworking electrode E3, error currents attributable to electrodedifferences are believed to appear when polarization is applied, whereinproviding the compensatory offset allows the working electrode E3 to bepolarized about its free corrosion potential (Ecorr) as measured withrespect to the reference electrode (RE), rather than about some otherpotential, thereby improving overall corrosion measurement accuracy.

A third measurement portion 103 of the exemplary device cycle shown inFIG. 7 employs detection of spontaneous noise with no externalexcitation for ECN type measurements. In this measurement mode, thedevice 2 measures sensed current (and voltage), and calculatesstatistical parameters based thereon, including mean, standard deviation(σ), and rms in certain embodiments, and further computes thesestatistics from statistical ‘moments’ of the data. Where used, thevoltage or potential noise is measured between the reference electrodeE2 and circuit ground, where the auxiliary and working electrodes E1 andE3 are effectively connected by the switching system 34 c to a virtualground. The statistical moments themselves may be computed from acomplete data set (e.g., many samples of voltages and currents measuredover a period of time), but such an approach would involve extensivecomputational overhead in the processor 22 and high memory usage. Inpreferred embodiments, therefore, a ‘running moment’ approach isemployed so as to require significantly less memory. In the illustratedimplementation, the processor 22 computes the first two statisticalmoments of the noise data for both current and voltage or for currentalone, from which the statistics for mean, standard deviation, and rmsare calculated, and used in the on-line electrochemical noise (ECN)measurements. The ECN is advantageously employed in the device 2 forcomputing a noise index or localized corrosion index value, wherein anyform of such localized corrosion index may be computed in the device 2which is indicative of the propensity of the electrodes 8 to localizedcorrosion attack in a given electrolyte. In one embodiment, adimensionless localized corrosion index number is computed, which, whenexceeding a certain level, indicates the possibility of localizedcorrosion attack occurring in a given installation. The device 2, in oneembodiment, provides the localized corrosion index value as the ratio ofthe current noise standard deviation σ_(i) to the current noise rmsvalue (rms_(i)), which value is between 0 and 1, wherein high σ_(i)values and low dc current values (usually seen with pitting attack) willresult in a high (near 1) values for the localized corrosion index. Lowuniform corrosion (low σ_(i) and low dc currents) on the other handcorrespond to localized corrosion index values near zero. In onepossible implementation, a threshold value of 0.3 can be used as awarning limit, over which a possible localized corrosion attack may beindicated to the user, although any suitable index (unitless orotherwise) and corresponding comparison values may be selected.

Current noise is sampled in the device 2 via the working electrode E3and a weighted average or running moment is computed, with the currentnoise statistics being used to compute the localized corrosion index. Inone embodiment, moreover, the voltage (potential) noise may likewise bemeasured using the voltage sensing circuitry of the probe interface 30and a second input channel to the A/D 26. In one preferredimplementation, the device 2 uses running moment calculations incomputing rms or standard deviations in deriving the localized corrosionindex or other statistical measure. In this manner, the device 2 doesnot need to store large amounts of data and the number of requiredcomputations in each device cycle is reduced. In one implementation, thenoise statistics are computed as running moments for each A/D sample andthe process repeats until a certain number of samples “n” have beenobtained, such as 1000 in one example. In this case, two momentvariables M1 and M2 are initialized to zero by the processing system 22,and a variable for n is set to 1. The processor 22 then sets theswitching system to the ECN configuration, and the sampled current andvoltage measurements are incorporated into running computations toupdate the moment values at each sample time. The following equationsprovide for updating the moments with xn being the present currentsample value and n being the present sample number (e.g., n ranges from1 through 1000 in this example):d=(xn−M1)M2=M2+(1/n)*(d ²(1−(1/n)−M2))M1=M1+(d/n)

In this implementation, moreover, similar computations are made forvoltage samples obtained concurrently with the current samples, wherethe processing system 22 computes the moving moment values M1 and M2 forthe voltage noise as well. Moreover, the above calculations arepreferably optimized for execution time and memory use, such as byprecalcualting certain common factors like (1−1/n) for each pass,wherein the calculations of M2 and M1 are done in the order indicatedabove for each sample cycle until the predefined number of readings(e.g., n=1000) have been obtained for both current and voltage readings.Thereafter, the current statistics may be computed as follows:Mean=M1Current standard deviation σ_(i)=(M2)^(1/2)Current rms value rms _(i)=(M1² +M2)^(1/2)Current noise index=(M2/(M1² +M2))^(1/2)

The processor 22 similarly computes like statistics for the voltagenoise and then computes the current corrosion noise I_(corrnoise) as:I _(corrnoise)=((B _(HARM) OR B_(USER))*σ_(i))/(In(10)*σ_(V))

In another possible embodiment, the processor 22 computes a localizedcorrosion index based on a standard deviation of sampled current signalsand on an rms of the sampled current signals, where the standarddeviation and rms are both based on the running moment calculation. Inthis implementation, the voltage signals need not be sensed, and thecorresponding voltage noise statistics need not be computed forlocalized corrosion measurement, thereby reducing the computational andmemory storage overhead for the processor 22. In this approach, themoments M1 and M2 are computed for the measured current noise (with noexcitation), and the current noise index I_(corrnoise) is computed as:I _(corrnoise)=σ_(i) /rms _(i)=(M2/(M1² +M2))^(1/2)

Another feature of the device 2 involves effectively shorting theauxiliary and working electrodes E1 and E3 by connecting these to avirtual ground of the probe interface system 30 during the ECNmeasurements. In one embodiment, the processing system selectivelyreconfigures the switching components U13-U16 as shown in the ECN entryof table 70 in FIG. 4, by which the auxiliary electrode E1 is connectedthrough resistors R54 and R58 and through the “0” state of switch U14 tothe inverting input of amplifier U10A providing a virtual ground, andthe working electrode E3 is connected through resistor R56 to thevirtual ground at the inverting input of U9A, as shown in FIG. 3A duringthe ECN measurements in the third measurement period 103 while theprocessor 22 performs the above measurements and calculations. For theECN measurement, the auxiliary and working electrodes E1 and E3 areeffectively shorted (e.g., with zero ohms therebetween) while stilloperating for measurement of current flowing between them via thecurrent to voltage converter in the sensing circuitry coupled with theworking electrode E3, including current limiting protection resistorR57. In this regard, the exemplary sensing circuitry 34 b includes thisresistance (e.g., about 1 KOHM in one embodiment) in the feedback loopof the amplifier U9A and is thus not seen by the flowing current.

Another advantageous feature of the device 2 is the adaptability foroperation as a stand-alone data acquisition and storage device, whichmay be loop powered via a 4-20 mA control loop 11 or may be batterypowered via battery 13 in FIG. 2, wherein the battery 13 may bechargeable by solar panels or other means. In this regard, theprocessing system 22 computes corrosion related values such as R_(S),corrosion rate, localized corrosion index, etc., as described above, ineach of a series of device cycles and stores the computed values in thenon-volatile memory 24 (FIG. 2) for subsequent retrieval by a user. Thedevice 2 is accessed by a user communications device (not shown) throughthe control loop 11 or by other wired or wireless means to allowdownloading of the accumulated corrosion data, for instance, using HARTor other suitable communications protocol(s). The device 2, moreover, isoperable to store one or more day's worth of computed corrosion relatedvalues, such as over 5 days worth of data at long device cycle times inthe illustrated embodiment. In this respect, for shorted cycle times,more data could be stored, such as several months or even years worth ofdata. This feature is advantageous in remote applications where thedevice 2 may be isolated from a distributed control system, and mayoperate on battery or solar power independently to acquire corrosioninformation for several days at a time, which data can then be read fromthe device 2 in a few minutes and thus stored in an external usercommunications device for transfer to a spreadsheet or to another systemfor further evaluation, wherein the battery 11 may be charged by solarpanels connected to the device 2 in certain implementations.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present invention, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described components (assemblies, devices,systems, circuits, and the like), the terms (including a reference to a“means”) used to describe such components are intended to correspond,unless otherwise indicated, to any component, such as hardware,software, or combinations thereof, which performs the specified functionof the described component (i.e., that is functionally equivalent), eventhough not structurally equivalent to the disclosed structure whichperforms the function in the illustrated implementations of theinvention. In addition, although a particular feature of the inventionmay have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular application. Also, to the extent that theterms “including”, “includes”, “having”, “has”, “with”, or variantsthereof are used in the detailed description and/or in the claims, suchterms are intended to be inclusive in a manner similar to the term“comprising”.

1. A loop-powered corrosion measurement device for measuring ormonitoring corrosion of a structure exposed to an electrolyte,comprising: a loop interface comprising a power conversion system toconvert power from a 4-20 mA loop to power the device; a probe interfacesystem with signal conditioning circuitry to interface with a pluralityof measurement electrodes situated in the electrolyte, the signalconditioning circuitry comprising: excitation circuitry operative toprovide excitation signals to the electrolyte via a first one of theelectrodes, and sensing circuitry operative to sense one or morecorrosion-related electrical signals via at least a second one of theelectrodes; a processing system operatively coupled with the probeinterface system to control the excitation signals provided to theelectrolyte and to compute at least one corrosion related value based onsignals sensed by the sensing circuitry; and an isolation barrierproviding galvanic isolation of the electrodes from the 4-20 mA loop. 2.The corrosion measurement device of claim 1, wherein the isolationbarrier comprises an isolation transformer coupled between the 4-20 mAloop and the power conversion system.
 3. The corrosion measurementdevice of claim 1, wherein the loop interface comprises an intrinsicsafety barrier with voltage and current limiting components.
 4. Aloop-powered corrosion measurement device for measuring or monitoringcorrosion of a structure exposed to an electrolyte, comprising:comprising a power conversion system to convert power from a 4-20 mAloop to power the device; a probe interface system with signalconditioning circuitry to interface with a plurality of measurementelectrodes situated in the electrolyte, the signal conditioningcircuitry comprising: excitation circuitry operative to provideexcitation signals to the electrolyte via a first one of the electrodes,and sensing circuitry operative to sense one or more corrosion-relatedelectrical signals via at least a second one of the electrodes; aprocessing system operatively coupled with the probe interface system tocontrol the excitation signals provided to the electrolyte and tocompute at least one corrosion related value based on signals sensed bythe sensing circuitry; and an isolation barrier providing galvanicisolation of the electrodes from the 4-20 mA loop; wherein the loopinterface comprises an intrinsic safety barrier with voltage and currentlimiting components; and wherein the intrinsic safety barrier comprisesa first stage with voltage and current limiting components coupled to anoutput of the isolation barrier, and a second stage with additionalvoltage limiting components coupled between the first stage and a loopcontroller circuit of the device.
 5. The corrosion measurement device ofclaim 4, wherein the intrinsic safety barrier further comprisesprotection resistors coupled to the electrodes.
 6. The corrosionmeasurement device of claim 3, wherein the intrinsic safety barrierfurther comprises protection resistors coupled to the electrodes.
 7. Aloop-powered corrosion measurement device for measuring or monitoringcorrosion of a structure exposed to an electrolyte, comprising: a loopinterface comprising a power conversion system to convert power from a4-20 mA loop to power the device, and an intrinsic safety barrier withvoltage and current limiting components; a probe interface system withsignal conditioning circuitry to interface with a plurality ofmeasurement electrodes situated in the electrolyte, the signalconditioning circuitry comprising: excitation circuitry operative toprovide excitation signals to the electrolyte via a first one of theelectrodes, and sensing circuitry operative to sense one or morecorrosion-related electrical signals via at least a second one of theelectrodes; and a processing system operatively coupled with the probeinterface system to control the excitation signals provided to theelectrolyte and to compute at least one corrosion related value based onsignals sensed by the sensing circuitry.
 8. A loop-powered corrosionmeasurement device for measuring or monitoring corrosion of a structureexposed to an electrolyte, comprising: a loop interface comprising apower conversion system to convert power from a 4-20 mA loop to powerthe device, and an intrinsic safety barrier with voltage and currentlimiting components; a probe interface system with signal conditioningcircuitry to interface with a plurality of measurement electrodessituated in the electrolyte, the signal conditioning circuitrycomprising: excitation circuitry operative to provide excitation signalsto the electrolyte via a first one of the electrodes, and sensingcircuitry operative to sense one or more corrosion-related electricalsignals via at least a second one of the electrodes; and a processingsystem operatively coupled with the probe interface system to controlthe excitation signals provided to the electrolyte and to compute atleast one corrosion related value based on signals sensed by the sensingcircuitry; wherein the intrinsic safety barrier comprises a first stagewith voltage and current limiting components coupled to an output of theisolation barrier, and a second stage with additional voltage limitingcomponents coupled between the first stage and a loop controller circuitof the device.
 9. The corrosion measurement device of claim 8, whereinthe intrinsic safety barrier further comprises protection resistorscoupled to the electrodes.
 10. The corrosion measurement device of claim7, wherein the intrinsic safety barrier further comprises protectionresistors coupled to the electrodes.
 11. The corrosion measurementdevice of claim 7, wherein the intrinsic safety barrier furthercomprises an isolation transformer.
 12. A corrosion measurement devicefor measuring or monitoring corrosion of a structure exposed to anelectrolyte, comprising: a power conversion system using a current of 20mA or less to power the device; a probe interface system with signalconditioning circuitry to interface with a plurality of measurementelectrodes situated in the electrolyte, the signal conditioningcircuitry comprising sensing circuitry operative to sense one or morecorrosion-related electrical signals via at least one of the electrodes;a processing system operatively coupled with the probe interface systemto compute at least one corrosion related value based on signals sensedby the sensing circuitry in each of a series of device cycles and tostore a plurality of computed corrosion related values in the device forsubsequent retrieval by a user; and a communications interfaceoperatively coupled with the processing system and operative tointerface with an external communications device to allow the user toconfigure the device or to retrieve stored computed corrosion relatedvalues from the device via the external communications device.
 13. Thecorrosion measurement device of claim 12, wherein the device includes aloop interface coupled to a 4-20 mA loop, and wherein the powerconversion system is operable to power the device using power from the4-20 mA loop.
 14. The corrosion measurement device of claim 13, whereinthe communications interface is operative to interface the device with acommunications device via the 4-20 mA loop.
 15. The corrosionmeasurement device of claim 12, further comprising a battery, whereinthe power conversion system is operative to power the device frombattery power.
 16. The corrosion measurement device of claim 12, whereinthe communications interface is operative to interface the device with awireless communications device.
 17. The corrosion measurement device ofclaim 12, wherein the communications interface supports HART protocolcommunications.
 18. The corrosion measurement device of claim 12,wherein the device stores at least one day's worth of computed corrosionrelated values.
 19. The corrosion measurement device of claim 12,further comprising an isolation barrier providing galvanic isolation ofthe electrodes from the 4-20 mA loop.
 20. The corrosion measurementdevice of claim 12, wherein the loop interface comprises an intrinsicsafety barrier with voltage and current limiting components.
 21. Thecorrosion measurement device of claim 12, further comprising anon-volatile memory, wherein the processing system stores the computedcorrosion related values in the non-volatile memory for subsequentretrieval by the user.